In recent years, there has been suggested a technology for detecting a lane mark such as a white line on a road such as an express highway by capturing an image of the road along which a vehicle travels with imaging means such as a CCD camera mounted on the vehicle and processing the captured image (for example, refer to Japanese Patent Laid-Open No. Hei 11(1999)-85999 (hereinafter, referred to as Patent Document 1)).
The white line detection device for a vehicle in Patent Document 1 extracts an edge point (negative edge point) where binary data changes from light (white) to dark (black) over the entire image area and an edge point (positive edge point) where binary data changes from dark (black) to light (white) by using a monochrome image (black and white binary image) of a traffic lane obtained by the camera mounted on the vehicle. Then, the white line detection device for a vehicle performs a Hough transform for the positive edge point and the negative edge point independently and combines their Hough transform results to detect a white line.
Furthermore, a road sign (lane mark or traffic sign) other than a white line such as, for example, a yellow line is occasionally used. In this situation, it is difficult to accurately detect the road sign from a monochrome image. Therefore, there has been suggested a technology for detecting a road sign using a color image (for example, refer to Japanese Patent No. 3333468 (hereinafter, referred to as Patent Document 2)).
When a road image is captured by a camera, however, the road may be partially shaded by a building around the road, for example, as shown in FIG. 3(a) or the road may be partially illuminated by the headlights of the vehicle in some cases. If the road image is captured in a situation where the road illumination state is partially different according to the light sources illuminating the road surface or the environment around the road as described above, for example, a shaded area is illuminated only by scattered light and a color feature value of pixel data varies. Therefore, the yellow line located in this area is not recognized as a line having the original yellow color. Accordingly, there has been a problem that it is difficult to appropriately detect a road sign even if a color image is used as in Patent Document 2 in some cases.